1. Field of the Invention
This invention relates to a compact and high-sensitivity gas sensor element for measuring chiefly a CO.sub.2 concentration in a gas phase and, in addition, NO.sub.x, and SO.sub.x concentrations. More particularly, it relates to a gas sensor element in which parts are joined not by mechanical pressing with the aid of bolts, etc. but by contact or bonding using a bonding agent to make the sensor element more compact and more sensitive in detecting gas concentrations. The gas sensor element of the invention ultimately achieves reduction in size and cost and increase in sensitivity of measuring instruments.
The present application is based on Japanese Patent Application No. Hei. 10-80684, which is incorporated herein by reference.
2. Description of the Related Art
In recent years gas sensors capable of measuring mainly a CO.sub.2 concentration and secondarily NO.sub.x and SO.sub.x in a gas phase have been developed to cope with the environmental pollution. Of these gas sensors, CO.sub.2 sensors are useful for conditioning control in a limited space, such as the space in a vehicle, a house or an office, or for CO.sub.2 detection and control in horticultural facilities, and NO.sub.x or SO.sub.x sensors are indispensable to environmental conservation against air pollution.
In these fields, various gas sensors of concentration cell type have recently been proposed, which comprise an alkali metal ion conductor having on each ends thereof a detecting pole, electrodes, etc. However, they have insufficient durability due to deterioration by reactions among constituting materials.
The inventors of the present invention previously proposed a compact and simple gas sensor utilizing an equilibrium reaction which is made of a lithium ion conductor (alkali metal ion conductor), e.g., LISICON, having provided on one side thereof a detection electrode mainly comprising lithium carbonate (alkali metal carbonate) in dissociation equilibrium with CO.sub.2 gas and on the other side thereof a solid standard electrode comprising lithium-containing complex oxides different in composition, for example LiFeO.sub.2 and LiFe.sub.5 O.sub.8. A CO.sub.2 gas sensor of this type is capable of precisely measuring the CO.sub.2 gas concentration in the surrounding atmosphere without using a standard gas.
In order to obtain a practical measuring efficiency with the above CO.sub.2 gas sensor, the alkali metal carbonate (detection electrode) in dissociation equilibrium with CO.sub.2 gas and the alkali metal ion conductor must be bonded with high strength while securing ion conduction, heat resistance (300.degree. C. or higher), and capability of absorbing the difference in thermal expansion coefficient between them. A mechanical bonding means such as bolts and nuts is conceivable but difficult because the CO.sub.2 gas sensor itself is as small as several millimeters. Besides, such a mechanical means would be a bar to size reduction of the sensor. The alkali metal carbonate and the alkali metal ion conductor could be bonded thermally, but such bonding requires heating at a considerably high temperature for a long time. In using, for example, lithium carbonate as an alkali metal carbonate, it melts on heating at temperatures above its melting point (720.degree. C.), resulting in complete disappearance of voids in the detection electrode that are essential to gas diffusion. Further, such heating induces a reaction between lithium carbonate and the alkali metal ion conductor, e.g., LISICON, to form a high-concentration lithium layer in the interface therebetween, which causes a drift of the electromotive force. Correction of such a drift requires a very long time of initial annealing.